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Paris Hilton's boyfriend Cy Waits was driving under the influence of marijuana when he was arrested with the celebrity socialite on the Las Vegas Strip last summer, according to a criminal complaint filed by prosecutors Thursday.

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Waits, 35, faces a felony charge of being under the influence of marijuana, and misdemeanor charges of possession of marijuana and driving under the influence. The former nightclub mogul could face up to four years in prison if convicted of the felony charge.

A toxicology report shows Waits had marijuana in his blood around the time of his arrest. Although drug possession is often a misdemeanor, it is a felony in Nevada to use or be under the influence of any illegal drug.

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Waits' attorney, David Chesnoff, said his client is not guilty. Chesnoff said he has not received the forensic evidence.

"We are looking forward to vigorously defending the matter," Chesnoff said Thursday. "At this point it is just an allegation."

Waits' Aug. 27 traffic stop made national headlines after he and Hilton were arrested.

Police said a small bag containing 0.8 gram of cocaine fell from Hilton's Chanel purse as she reached for a tube of lip balm after the traffic stop.

Hilton pleaded guilty in September to misdemeanor cocaine possession and obstruction charges. She is serving a year of probation and was banned from two Wynn resorts after the arrest, and Waits was dismissed as a partner in the venue's nightclub.

The couple caught a police officer's attention as Waits was driving a black Cadillac Escalade on the Strip at 11:21 p.m., according to a police report. The officer said he smelled a "vapor trail" of marijuana smoke coming from the SUV.

Waits told the officer he had just finished smoking. His pupil size appeared normal, but his breath smelled of alcohol, his eyes were bloodshot and he appeared to wobble when he stood, according to a police report.

Waits passed a test in which the officer asked him to follow a finger with his eyes. But Waits failed a walk-and-turn test and a one-leg-stand test, showing poor balance in both, according to the report.

A vehicle search turned up the remains of a moist marijuana cigarette.

Chesnoff, who also represented Hilton in her criminal case, said he would challenge the legality of the stop. He said the officer did not have probable cause that Waits was driving while impaired. He also questioned why the officer continued his investigation after Waits passed the eye test.